Conventionally, for the purpose of eliminating the repaired work for the surfaces of continuous castings, there has been proposed a technique of oscillating a vertical mold for reducing or preventing positive segregations at oscillation mark trough portions on the surfaces of the castings, particularly, in casting stainless steel (SUS 304). For example, Japanese Patent Laid-open No. hei 2-290656 discloses a technique where, in a continuous casting mold of a type forming a casting space with two pairs of mold wall surfaces, a pair of the mold wall surfaces are relatively separated from each other only for a negative strip time zone in vertical oscillation or for a mold descending time zone.
This technique is recognized to be considerably effective as compared with a case of using only simple vertical oscillation. However, as a result of an experiment, it is seen that this technique is not much effective in a case where the oscillation frequency &lt;f&gt; of the mold is small. Further, in the above technique, the consumption of mold powder is reduced, thereby causing the breakout due to sticking. Accordingly, this is an obstacle for obtaining a stable casting.
Conventionally, the mechanism and cause of segregations at oscillation mark trough portions are considered as follows: negative pressure is generated within a liquid phase lubricating film between the mold and the solidified shell due to oscillation of the mold; and due to this negative pressure, the non-solidified and concentrated molten steel between dendrites of the solidified surface layer permeate onto the surface of the shell.
However, as a result of the examination on the segregated portions of the castings by the present inventors, it was discovered that segregation is generated in accordance with such a mechanism that the continuous growth of the solidified shell is obstructed by breaking of the shell due to a tensile force applied thereto and by buckling due to a compressive force. Thereby the concentrated liquid or molten metal flows out from the broken portions or buckled portions of the shell to the surface of the shell. Accordingly, to prevent the segregation, it is necessary to prevent the breaking or the buckling of the shell at the beginning of the solidification, that is, to simultaneously reduce the tensile force and the compressive force applied to the shell.
An object of the present invention is to provide a process of withdrawing the continuous castings wherein, even in the low cycle condition that the oscillation frequency &lt;f&gt; of the mold is small, the segregation at oscillation mark trough portions on the surfaces of the castings are significantly reduced to the degree equivalent to that in the high cycle condition, and also to provide stable castings.